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Subject: Re: [boost] [tti] review
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-07-18 18:51:18

On 7/17/2011 10:00 PM, Nathan Ridge wrote:
>> From: jeffrey.hellrung_at_[hidden]
>> On Sun, Jul 17, 2011 at 2:48 PM, Nathan Ridge<zeratul976_at_[hidden]>wrote:
>>>> On 7/17/2011 4:10 PM, Edward Diener wrote:
>>>>> I agree with Jeff's suggestion that the matching metafunction should
>>> take a metafunction that defaults to is_same rather than always testing for
>>> type equality (and now I see why he was also suggesting that in the Type
>>> Traits Extension review :).
>>>> I will look further into adding this but I strongly feel that I should
>>>> not drop the simple functionality currently provided, where the end-user
>>>> can just pass an actual type.
>>> As a casual Boost user, I would welcome this. I would find TTI very
>>> useful, but I do not know MPL beyond the very basics, and I'd rather not
>>> have to learn it just so that I can use TTI. I think others may be in
>>> the same position.
>> My initial response is, grab a beer and go learn about Boost.MPL lambda
>> expressions, you'll be happy you did (at worst, you're +1 drunk beer).
>> Okay, my secondary response: I don't think it's really that bad to write
>> has_type_xxx< T, boost::is_same< boost::mpl::_1, U> > // my proposed query
>> rather than
>> has_type_xxx< T, U> // current query
>> (a) How often do you *really* want to check that T::xxx is precisely U? I
>> think, much more frequently, you'll just want has_type_xxx<T>. And I'm
>> *not* proposing to get rid of *that*.
>> (b) To me, the former states more explicitly how U ties into the query.
>> (c) With the current has_type_xxx semantics, it's going to get onerous one
>> metaprogramming day when boost::is_same< boost::mpl::_1, U> just isn't want
>> what you want. You want to check if T::xxx is an object type. Or whether
>> it's const-qualified. Or any of a number of other properties in
>> Boost.TypeTraits. Or :: gasp :: whether T::xxx itself has a nested type yyy
>> :)
> I suppose, as long as the TTI docs show some clear examples of how to use
> the MPL lambdas in the most common cases, rather than just saying "any
> MPL lambda can be used here" and referring you to the MPL docs, it should
> be fine. I just don't think users should have to go off and learn about
> MPL lambdas by reading some separate documentation, just to be able to
> make a simple use of this TTI feature.
> Apart from that concern, I *do* see the value in being able to check for
> other things besides the types being the same, and I think it would be
> a useful addition.

I will probably be adding Jeffrey Hellrung's suggestion to TTI. If I do
I will give examples which show how to use boost::mpl::lambda to pass a
metafunction which depends on placeholders as an argument to the
HAS_TYPE generated metafunction. For further knowledge of what lambda
can do, the metaprogrammer should read the MPL documentation.

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